Summary: The retinal pigment epithelium (RPE) is the major barrier between the outer neural retina and the choroid, forming part of the blood retina barrier (BRB). Damage to the RPE cell and to the organization of the RPE sheet impairs the BRB, as seen in AMD, uveitis, retinal degenerations, and retinal detachment. Additionally, damage to the RPE can occur following subretinal surgery or subretinal injections. Extensive work by others with non-ocular epithelial monolayers indicates that such tissues respond to stress and restore their structure and function through a small number of protective mechanisms. Our preliminary experimental observations suggest that the RPE similarly responds to damage or disease, restoring the BRB via definable protective mechanisms. From these observations, we hypothesize: a) There are a limited number of responses that the RPE takes to restore the BRB; b) Each kind of response results in a unique pattern of RPE cell death; c) Each response type differs, thus modeling BRB repair must be flexible (Fig 1). For these reasons, we propose here to test for specific mechanisms that the RPE uses to respond to insult and to restore the BRB following chronic mild, moderate, and severe injury. While restoration of barrier functions has been investigated in Drosophila wings, lung alveoli, and throughout embryology, it has not been studied in the RPE sheet. Most research on epithelial monolayers, whether in RPE or in non-ocular tissue, uses 2- dimensional (2D) static or time-lapse motion photomicroscopy to study damage responses. In this project, we adapt clever software and mathematical tools from outside vision research that use quantitative spatiotemporal (4D: 3D in space & 1D in Time) dynamics to explore the damage responses of individual RPE cells and of the RPE sheet. Significance: Completion of these Aims will increase our understanding of spatiotemporal dynamics and biomechanics of repair of the BRB after short- or long-term, low- to high-level toxic insults to RPE cells. This new understanding of mechanisms will allow us to correct the loss of essential barrier function in blinding diseases by initiating early and inexpensive interventions.